This invention relates to a method and apparatus for protecting an antenna window and, more particularly, to a method for covering an elongated antenna window that is disposed in a nose cone of a reentrytype missile.
Present practice for covering an antenna window of a nose cone of a reentry-type missile for protecting components internal the missile employs one of two general approaches. One scheme uses a plate fabricated from a homogeneous dielectric material to cover the window. Typical dielectrics, such as monolithic ceramics, are brittle and are subject to damage due to stresses experienced during flight, especially when the missile is maneuvering during reentry. In another approach, a ground plane cover, such as may be fabricated from a carbon composite material, is disposed over the antenna window. The ground plane cover includes a plurality of discrete smaller openings having dielectric material disposed therein. An example similar to the latter is shown in U. S. Pat. No. 4,570,166 -- Kuhn et al, assigned to the instant assignee, in which the solid metal wall of the nose cone is perforated to form a grid array of windows, each of which has a respective dielectric plug member fitted therein.
A four-directional (4D) triangular fiber arrangement is described in a DTIC report ADB049350 entitled "Boron Nitride -- Boron Nitride Composite Material" by Potter and Place. FIG. 4 of the Potter and Place report illustrates a cylindrical configuration having three triangularly related fibers disposed in a plane perpendicular to the central axis of the cylinder and one fiber disposed in a plane parallel to the central axis of the cylinder. This orientation of the Potter and Place 4D fiber arrangement provides rigidity against the curve of the cylindrical material being flattened out, i.e. against shear in a plane perpendicular to the central axis. Although the 4D triangular fiber arrangement of Potter and Place may be used as an antenna cover, when disposed so that the one fiber is in a plane parallel to the central axis of the nose cone, there would be lower rigidity or stiffness in the plane of expected shear than that contemplated by use of the present invention, i.e. in a plane parallel to the central axis of the nose cone.
An uninterrupted 4D fiber configuration for a portion of a nose tip of a missile is described in U.S. Pat. No. 4,400,421 -- Stover. Details of the structure of the nose cone 28 are not shown or described nor is an antenna window or cover shown in nose cone 28 of U. S. Pat. No. 4,400,421. The orientation of the fibers of the 4D configuration of U.S. Pat. No. 4,400,421 in a nose tip of a missile is substantially the same as that contemplated by an antenna window cover when operationally disposed in an orifice of a nose cone in accordance with the present invention.
Although at column 2, lines 55-56 of U. S. Pat. No. 4,400,421 it states that, "The 4D/3D construction is particularly adapted for use in the fabrication of nose cones," the detailed description of the use of the 4D/3D configuration at column 6, lines 3-19, clearly indicates that it is the nose tip which includes the 4D/3D configuration that may then be attached to the nose cone as described at column 5, lines 37-40.
For applications contemplated by the present invention, a large antenna window aperture is generally longer than it is wide, such as oval or elliptical, and may have an aspect ratio of at least about 1.5 to 1 with the longer axis disposed substantially along a meridian of the surface of the nose cone and the shorter axis perpendicular or transverse thereto. In an operational environment, and especially while maneuvering during the reentry phase of flight, shear stresses along the longer axis tend to be greater than along the shorter axis and thus it would be desirable to provide an antenna window having more structural material for support in the long axis than in the short axis of the elongated window.
Accordingly, it is an object of the present invention to provide a method for disposing more structural reinforcement fiber material for support in an expected plane of higher shear of a composite material cover for an elongated antenna window.
Another object of the present invention is to provide a method for protecting the environment interior a cavity of a nose cone of a missile.
Still another object is to provide a cover for an elongated antenna window of a nose cone wherein the cover includes greater reinforcement along the longer axis than the shorter axis.